Patients with coronary artery disease (CAD) have impaired endothelium-dependent vasodilation due in part to an increase in superoxide anion levels. p22phox, an essential component of the NADPH oxidase, is thought to play a critical role in the generation of superoxide anions in the vessel wall. The C242T polymorphism, located in the potential heme-binding site of the p22phox gene, has recently been reported to confer a protective effect on CAD risk in a Japanese study population. By using a single-stranded conformation polymorphism analysis, we first investigated the allelic frequency of the C242T polymorphism in the U.S. population. In a population of 176 patients with no known history of heart disease (control), the prevalence of the allele (heterozygous + homozygous) was 59.7%, which was significantly higher than previously reported in the Japanese population. We next analyzed 246 patients (82% Caucasian) undergoing angiography for diagnosis of CAD. In 149 patients with any angiographic evidence of atherosclerosis, the prevalence of the C242T polymorphism was 64.4%, whereas in 97 patients with angiographically smooth coronary arteries, the prevalence was 55.6% (Chi-square=N.S.). We next investigated the effect of the polymorphism upon microvascular endothelial function in a subset of 140 patients undergoing angiography using intracoronary acetylcholine (ACH) (3-100 mcg/min). As measured using Doppler flow wire, vasodilation in response to ACH was similar in patients with and without the C242T allele. Thus, the prevalence of the C242T polymorphism, which is higher in the U.S. than in the Japanese population, does not confer protection from endothelial dysfunction of CAD.